1. Field of the Invention
This invention relates to eddy current sensors. More specifically, the invention is a radially focused eddy current sensor for detecting longitudinal flaws in a metal tube.
2. Description of the Related Art
The inspection of longitudinal welds in metallic tubular structures is a major concern in the nuclear power industry where critical pressure vessels are typically welded together in longitudinal sections. Corrosive environments can speed the degradation of these welds and can limit access thereto for inspection of the welds.
To help overcome these obstacles, eddy current probes have been developed that simply require positioning and moving an eddy current coil in a tubular piece being inspected. However, conventional eddy current probes are based on impedance measurements of the piece being inspected. Such measurements require complicated instrumentation and often provide flaw signatures which are difficult to interpret. This is especially true for differential measurements necessary for most tubing inspections. In addition, the driving fields of the conventional eddy current probe are not focused so that the coupling between the primary field of the probe and the flaw is normally weak. The impedance measurements of eddy current probes are also extremely sensitive to changes in the conductivity and permeability of the piece being inspected. Small changes in these parameters, which are common near weld joints, can therefore disguise the signal due to a fatigue crack.